A solder to be used for bonding of electronic parts is advancing to be lead-free from the viewpoint of environment, and has now been employed a solder powder containing tin as a main component. As a method for obtaining a fine metal powder such as the solder powder, there have been known, in addition to the atomizing method such as the gas atomizing method and the rotary disc method, etc., the melt spinning method, the rotary electrode method, the mechanical process and the chemical process, etc. The gas atomizing method is a method in which a metal is melted by an induction furnace or a gas furnace, the molten metal is flowed down from a nozzle at the bottom of a tundish, and a high pressure gas is blown thereto from the circumference to powderize the metal. The rotary disc method is also called as the centrifugal force atomizing method, and is a method to prepare fine powder by dropping a molten metal on a disc rotating with a high speed to apply a shearing force to the tangential direction to fracture the metal.
On the other hand, finer pitch of bonding parts is also progressing with miniaturization of electronic parts, and solder powder having a finer particle size is desired, so that improvement in technologies for such a finer pitch has also been actively carried out. For example, as a technique in which the gas atomizing method has been improved, it has been disclosed a method for manufacturing metal fine powder in which a molten metal in a gas-entrained state is jetted from a nozzle and a high pressure gas is blown from the circumference of the nozzle against the metal (for example, see Patent Document 1.). According to the method disclosed in Patent Document 1, by entraining the gas into the molten metal when it passes through the nozzle, the molten metal has already been separated at the time of tapping from the nozzle whereby finer powder can be produced.
In addition, as a technique in which the rotary disc method has been improved, it has been disclosed a preparation method of metal fine powder in which a mesh is arranged to a rotary member as a means for adjusting a size of the metal fine powder, and a molten metal is scattered through the mesh (for example, see Patent Document 2.). According to the method disclosed in Patent Document 2, fine metal powder can be formed with good efficiency as compared with the conventional rotary disc method.
Further, it has been is disclosed a solder powder which is obtained by the wet reduction method, that a yield of the solder powder having an average particle size of 5 μm or less is extremely high (for example, see Patent Document 3.). This solder powder is a ternary solder powder which consists of a metal particle of a center core, a covering layer encapsulating the center core, and an outermost layer encapsulating the covering layer to improve wettability of a paste for solder or strength required for a solder bump. This solder powder consists of a metal particle in which all the three kinds of metals are contained in one particle, so that the composition is more uniform as compared with a solder powder in which different kinds of single-metal powders are simply mixed. Also, it has a structure in which an intermediate layer and an outermost layer are successively coated over a center core in this order depending on an ionization tendency of metal elements forming the respective layers, so that a process of reducing a metal ion to precipitate a powder is not so complicated whereby it is also excellent in mass productivity.